[userbinator]

while Facebook is probably at least 100 million lines of code at this point

...which, if I were to guess, consists of a significant amount of "do nothing" or otherwise useless abstractions that either were put there dogmatically or once served a purpose but no longer does. This is reasonably prevalent in Enterprise software but I imagine the much higher churn in Facebook code also increases the amount of cruft that gets thrown in or otherwise left behind.

Search HN or the Internet in general for mentions of Facebook's app being bloated and you'll find that even non-technical users are at least somewhat surprised by this gross inefficiency.